|Field emission display (FED)|
|Thursday, 05 October 2006|
The liquid crystal display (LCD) provides an alternative which is not subject to some of the disadvantages of the CRT. A liquid crystal display device has a number of advantageous features including light weight, reduced thickness and low power consumption. However, the LCD has many inherent limitations that render it unsuitable for a number of applications. While LCD devices may be lighter in weight with lower power consumption relative to CRT devices, they tend to provide poor contrast with a limited angular display range. LCDs offer only a limited angular display range. Color LCD devices consume power at rates incompatible with extended battery operation. Lastly, a color LCD type screen tends to be far more costly than an equivalent CRT. Moreover, the response time of an LCD is dependent upon the response time of the liquid crystal to an applied electrical field, and the response time of the liquid crystal is relatively slow. Such shortcomings limit the use of LCDs in many applications such as high-definition TV (HDTV) and large displays. Plasma display panel (PDP) technology is more suitable for HDTV and large displays. However, a PDP consumes a lot of electrical power. Further, the PDP device itself generates too much heat. As the plasma display panels (PDP) with a large area and capable of emitting light automatically are merchandised and produced in a mass scale in the recent years, the technology for developing the plane displays with low cost and large area is advanced in a surprising speed. A great deal of research effort has be devoted in recent years to developing a display which can replace the conventional cathode may tube (CRT) and overcomes some of the limitations of conventional LCDs and PDPs. A field emission display (FED) has come into the spotlight as a next-generation display which has the advantages of a cathode-ray tube (CRT) and liquid crystal display (LCD). FEDs offer the prospect of flat panel displays which are superior to LCD screens in brightness, colour rendition, response time and operating temperature range.
A field emission display (FED) is a low power, flat cathode ray tube type display that uses a matrix-addressed cold cathode to produce light from a screen coated with phosphor materials. The principle of the field emission display is similar to that of the traditional cathode ray tube display. Field emission displays, like cathode ray tubes, display a color image by emitting light of a predetermined color through the bombardment of electrons onto a field emitter array (FEA) coated with phosphor. They both emit electrons to hit the fluorescent medium on a substrate in vacuum. Electron emission includes field electron emission, secondary electron emission, and photoelectric emission, as well as thermionic emission. A cold cathode is the cathode that performs electron emission by field electron emission, which occurs due to a tunnel effect when a strong electric field is applied to the vicinity of the surface of a substance to lower the potential barrier on the surface. The cathode ray tube display emits electron beams by a single electron gun and controls the direction of the electron beams by using a polarization plate. Instead, the field emission display is composed of hundreds of thousands of active cold emitters, each of which corresponds to a pixel independently, so no polarization plate is needed. Field emission displays (FEDs) apply a strong electric field from a gate to a field emitter disposed on a cathode layer at regular intervals, thereby emitting electrons from the field emitter, colliding the electrons with a phosphor material of an anode layer, and emitting light. The cold cathode electron source is broadly divided into a field emission electron source and a hot electron type electron source. The former includes a spindt type electron source, a surface conduction type electron source and a carbon nanotube type electron source. The latter includes an MIM (metal-insulator-metal) type electron source stacked with metal-insulator-metal and an MIS (metal-insulator-semiconductor) type electron source stacked with metal-insulator-semiconductor. When displaying an image in the field emission display, a driving method called a line sequential scanning scheme is used standardly. Display in each of the frames is performed for each scan line (horizontally). All the cold cathode electron sources corresponding to the number of data lines on the same scan line are operated at the same time.
A field emission display typically includes an electron emission tip configured for emitting a flux of electrons upon application of an electric field to the field emission device. An array of miniaturized field emission devices can be arranged on a plate and used for forming a visual display on a display panel. The emission tip is specifically shaped to facilitate effective emission of electrons, and may for example be conical, pyramidal, or ridge-shaped in surface profile, or alternatively the tip may comprise a flat emitter surface of low work function material. In the initial FEDs, the emitters were formed of what are referred to as Spindt-type metal tips (or microtips). A main material of the metal tips is molybdenum. In a field emission display, several hundreds to thousands of micro tips or carbon nanotubes (CNTs) per pixel are provided as an electron emission source on a back plate of FED, and a phosphor layer emitting a light by an electron from the electron emission source is formed on a front plate of FED. To generate the field needed for release of electrons, a matrix of switchable row and column electrodes is typically provided, in addition to the anode, and in this way pixels can be individually addressed. In order for the electron to travel in a FED, most FEDs are evacuated to a low pressure in order to provide a log mean free path for the emitted electrons and to prevent contamination and deterioration of the microtips. The resolution of the display can be improved by using a focus grid to collimate electrons drawn from the microtips. Field emission displays include an array of pixels, each of which includes one or more substantially emitter tips. The array of pixels of a field emission display is typically referred to as a field emission array. Each of the emitter tips is electrically connected to a negative voltage source by means of a cathode conductor line, which is also typically referred to as a column line. Another set of electrically conductive lines, which are typically referred to as row lines or as gate lines, extends over the pixels of the field emission array. Row lines typically extend across a field emission display substantially perpendicularly to the direction in which the column lines extend. As electrons are emitted by emitter tips and accelerate past the row line that extends over the pixel, the electrons are directed toward a corresponding pixel of a positively charged electro-luminescent panel of the field emission display. As electrons impact a pixel of the electro-luminescent panel, the pixel is illuminated.
The field emission devices can be classified into a diode-type, a triode-type, or a tetrode-type by electrode. Among various types of field emission devices, a triode field emission device is used most popularly. Triode field emission devices are used for electron guns and amplifiers as well as FEDs. A filed emission display (FED) with a triode structure consists of anode, cathode and gate electrode to achieve high illumination by applying a high voltage and a low current. The triode elements include the cathode (field emitter site), the anode (cathodoluminescent element) and the gate (grid). Where the FED employs a triode structure including a cathode, an anode, and a gate electrode, cathode electrodes and field emitters are formed on a rear substrate, and gate electrodes are formed on the cathode electrodes and emitters with an insulating layer interposed therebetween. Further, an anode electrode and phosphor layers are provided on an inner surface of a front substrate. In such FEDs, an insulation layer and gate electrodes are provided on cathode electrodes, holes are formed to expose the cathode electrodes through the insulation layer and gate electrodes, then a carbon nanotube electron emitting layer is formed within the holes and on the cathode electrodes. The insulating layer provides support for the grid and prevents the breakdown of the voltage differential between the grid and the base plate. In a diode type FED, a stripe-shaped cathode electrode is formed on a surface of a first substrate and a stripe-shaped anode electrode is formed on a surface of a second substrate, and the cathode electrode is orthogonal to the anode electrode at a distance. At the intersection of the cathode electrode and the anode electrode through vacuum, voltage up to 10 kV is applied to emit an electron between the cathode electrode and the anode electrode. The electron is made to get to a phosphor layer put to the anode electrode to excite the phosphor, and then light is emitted to display an image. In the a tetrode type FED, a plate-shaped or filmy convergence electrode is formed between a gate electrode and an anode electrode of a triode-type FED. With the convergence electrode provided, an electron emitted from an electron emission portion is converged with respect to each dot to excite a phosphor layer put to an anode electrode, and then light is emitted to display an image.
As mentioned above, a field emission display has an electron emission portion that emits an electron, which is formed on a cathode electrode. The field emission device may have a gate electrode over the cathode electrode through an insulating film. Various structures are proposed for the emission device. Specifically, there are a spindt (spint) type field emission device, a surface-type field emission device, an edge-type field emission device, and MIM (metal-insulator-metal). A spindt type FED includes a substrate, a cathode electrode of an electron emission unit formed thereon having a substantially conical shape, and a gate electrode of a lead-out electrode stacked on a substrate around the cathode electrode having an insulating layer therebetween. In the spindt type FED, a voltage is applied between the cathode electrode and the gate electrode in a vacuum to thereby produce a high electric field therebetween. As a result, electrons are emitted from a tip end of the cathode electrode through the electron emission mechanism in an electric field. The spindt type field emission device has a conical electron emission portion formed on a cathode electrode. The advantages of such structure include the high electron drawing efficiency since the electron emission portion is arranged in the vicinity of the center of the gate electrode where the electric field is most concentrated, the possibility to draw a pattern of an arrangement of the field emission device with accuracy to make it easy to optimize an arrangement of distribution of electric field and in-plane uniformity of drawn current is high, and the directivity of electron emission is regular, compared to the other field emission device. The edge emission type FED is configured such that field electrons are emitted from an emitter electrode to an anode electrode, wherein the emitter electrode is formed above a gate electrode via an insulating layer in a substantially flat sheet shape and an electric field is generated between the gate electrode and the emitter electrode to thereby cause the emitter electrode to emit electrons. The edge emitter electrode for emitting electrons can be formed substantially in a flat sheet shape, and the electrons are emitted by an electric field generated between the gate electrode and the emitter electrode. The edge emitter type FED having such characteristics is applied to a field of a planar type multipole vacuum tube, and may be applied in a flat panel display by being plurally arrayed on a plane. Further, the edge emitter type FED has superior responsiveness, brightness, environment resistance, and the like, as compared with a liquid crystal display.
A nanotube, and more specifically a carbon nanotube (CNT), is known to be useful for providing electron emission in field emission devices, such as cold cathodes that are used in a field emission display. The Spindt-type field emission emitter, which is generally used for field emission displays, uses a micro tip as an emitter for emitting electrons. The emitter has a problem in that the life span of a micro-tip is shortened due to atmospheric gases or a non-uniform field during a field emission operation. Moreover, with such a conventional metal emitter, a work function must be decreased to decrease a driving voltage for field emission, but there are limitations. To overcome this problem, field emission arrays using carbon nanotubes which have a substantially high aspect ratio, excellent durability due to their structure and excellent electron conductivity, as an electron emission source, have been developed. Nanostructures such as carbon nanofibers, cabon nanotubes, carbon nanohorns and nanowires containing Si, Ge, an alloy of III and V-group elements (e.g., GaAs, GaP, InAs/P) or an alloy of II and VI-group elements (e.g., ZnS, ZnSe, CdS, CdSe) have good mechanical strength, heat- and electro-conductivity and chemical stability. When used as an emission source of a field emission array in an electronic device, the nanostructure is capable of increasing the work function and lowering the driving voltage due to its high aspect ratio, as compared to a conventional field emission source. The use of a carbon nanotube as an electron emitter has reduced the cost of a field emission device, including the cost of a field emission display. Carbon nano tubes are anticipated to be an ideal electron emission source since they feature a low work function, the resultant electron emission source can be driven by applying low voltages, and the method of fabricating the same is not complicated. They will thereby offer advantages to realize a large size panel display. Carbon nanotube field emission display (CNT-FED) is one kind of FED with matrix driving. Each pixel has a cathode, lower plate, with a layer of nanotube for electron emission source and an anode, upper plate, with a transparent electrode for electron attraction. The cathode accelerates electrons to bombard fluorescent material for fluorescence. Arrangements of FEDs of pixels show images. Field emission displays employing CNTs instead of micro tips as electron emitters are far more advantageous than cathode ray tubes in terms of view angle, definition, power consumption, and temperature stability.
The quality and sharpness of an illuminated pixel site of the display screen is dependent upon the precise control of the electron emission from the emitter sites that illuminate a particular pixel site. To form a desired image, electron emissions may be initiated in the emitter sites for certain pixel sites while the adjacent pixel sites are held in an off condition. For a sharp image, it is important that those pixel sites required to be isolated remain in an off condition. The performance of components in field emission displays is usually impacted by a variety of conditions. FED devices rely upon a predetermined relationship between current utilized to drive illumination and the emission characteristics of a pixel. The FED devices are usually driven with a predetermined voltage designed to result in a particular current that produces a particular display intensity. Field emission display are less tolerant to particle shedding from the faceplate than CRTs and, thus, excellent and repeatable adhesion and faceplate integrity are required. The cathode of the field emission display is in very close proximity to the faceplate and is sensitive to any electronegative chemicals arriving on the cold cathode emitter surfaces, which could absorb them and increase the value of the emitter work function. FED vacuum tubes may contain minute amounts of contaminants which can become attached to the surfaces of the electron-emissive elements, faceplates, gate electrodes, focus electrodes, (including dielectric layer and metal layer) and spacer walls. These contaminants may be knocked off when bombarded by electrons of sufficient energy. Thus, when an FED is switched on or switched off, there is a high probability that these contaminants may form small zones of high pressure within the FED vacuum tube. In the configuration of the spindt type FED, a distance between the emitter electrode and the gate electrode is determined by a hole size provided in a resist pattern so that it is necessary to enhance accuracy in a lithography and an etching process in order to reproducibly and uniformly form the emitter electrode of an element for emitting a plurality of electrons.
Field emission display (FED) technology has been proposed as a display technology that enjoys the advantages of allowing for wide viewing angles as well as being thin and light weight. The field emission display has the advantage of high image quality found with the conventional cathode ray tube display. Also, the field emission display has advantages of high yield, fast reacting time, good performance in displaying coordination, having high brightness, light and thin structure, wide range of color temperature, high mobile efficiency, excellent distinguishability of tilted direction, etc. in comparison with the conventional liquid crystal display that has the disadvantages of blurred view angle, limited range of usable temperature, and slow reacting time. Moreover, the field emission display emits light spontaneously. Field emission display has not only soft picture, rapid reaction, and clear brightness like CRT, but also possesses characteristics of lightness of flat display and low performance consumption. FED has advantages of light weight and thin profile, like liquid crystal display (LCD), and advantages of high brightness and self luminescence, like cathode ray tube (CRT). The image quality of the field-emission display is similar to that of the conventional cathode ray tube (CRT) display, while the dimension of the field-emission display is much thinner and lighter compared to the cathode ray tube display. Being self-illuminant, field emission display does not require a back light source like the liquid crystal display. In addition to the better brightness, the viewing angle is broader, power consumption is lower, response speed is faster, and the operation temperature range is larger. Through the construction of a high efficiency fluorescent film, the field emission display provides outstanding brightness performance even outdoors so it is thought as a quite competitive display panel and is even likely to replace the liquid crystal display. Field emission devices are used in a number of different applications, including displays, e-beam lithography, chemical analysis and space propulsion.